Environmental pollutants are constantly subjected to sunlight and may be photodegraded to less toxic chemicals, or converted to more toxic chemicals or highly reactive species that may take part in further chemical reactions. We are interested in the (photo)reactions involving free radicals and/or singlet oxygen of nitrite, nitrate, nitric oxides NOx and humic acids. UV radiation leads to radical dissociation of HNO2 yielding NO and OH radicals and the generation of a small amount of singlet oxygen (1O2). We have studied the nitrite/UV photo-oxidation of organic and inorganic substrates. Using the EPR spin trapping technique, we have provided spectroscopic evidence for the formation of NO2, NO and OH radicals, and characterized for the first time some of the organic and inorganic radicals occurring during this photolysis. Although the OH radical is the most reactive oxidant formed during nitrite photolysis, other transients like nitrogen oxides, singlet oxygen or nitrogen peroxy anions are also generated and may participate in oxidation processes. Nitric oxide (NO) is an important air pollutant and is also produced photochemically by the action of sunlight on nitrite. Nitric oxide is also generated in vivo where it functions as a neurotransmitter and vasodilating agent. We have been developing trapping agents for the detection and determination of nitric oxide. Our initial efforts were focused on the aci form of nitromethane, to which nitric oxide adds readily affording an EPR detectable spin adduct. More recently we have studied several related nitroalkanes in order to determine their suitability to trap NO in aqueous solutions. We have also shown that aquatic humics generate both free radicals and singlet oxygen upon UV irradiation. However, the photochemical properties of humics from different sources such as rivers, ocean, and soil-based samples, can differ quite considerably. These findings are important for a better understanding of the photo-oxidative processes that occur in the natural environment.